1. Field of the Invention
The present invention relates to a controller for controlling the movement of a movable body such as a mirror of an optical switch composed of a tilt mirror for which a MEMS technique is used; and more particularly to an optical switch controller and a movable body controller capable of reducing residual vibration at movement control of the movable body and performing the movement control accurately.
2. Description of the Related Art
Recently, traffic has been significantly increasing with rapid spread of the Internet. A wavelength division multiplexing (WDM) system is available as a system to construct a large capacity optical communication network dealing with the increase of traffic. A basic optical network constructed with a WDM system is provided with an optical cross-connect (OXC) system and an optical add/drop module (optical add/drop multiplexer (OADM)). An optical node constituted by an optical cross-connect system and an optical add/drop module uses an optical switch such as a tilt mirror to which a micro-electromechanical system (MEMS) technology is applied.
A tilt mirror to which a MEMS technique is applied is constituted by a micro mirror structure and an electrical circuit integrated with each other, and allows light inputted from a plurality of ports to output from given ports by switching ports. The port switching allows optical exchange for switching optical signals of a plurality of systems on optical transmission lines to different systems.
FIG. 24 shows the structure of an optical switch. This drawing shows a side view of a tilt mirror constituted as an optical switch by applying a MEMS technique. In the tilt mirror 120 shown in the drawing, a mirror 121 a surface of which is a reflecting surface is capable of swinging, in the directions shown by arrows in the drawing, with the center axis 122 as the center. Swing operation of the mirror 121 switches outgoing angles of incident light A to given angles (a1, a2, and a3 directions) in both directions with reference to the horizontal level in order to allow the incident light to output at the given angles. The notation a1 denotes the minus angle region, and the notation a3 denotes the plus angle region. The tilt mirror 120 is provided with electrodes formed like the teeth of a comb when viewed from the plane, thus being a MEMS mirror, shaped like the teeth of a comb, in which the angle of the mirror 121 can be changed in all angle directions continuously, which is not illustrated. A technique related to such a three-dimensional MEMS optical switch is disclosed in, for example, non-patent document 1 described below.
The mirror 121 provided in the tilt mirror 120 is electrically grounded (GND), and one end of the mirror 121 faces a plus driving electrode 123a, and another end of it faces a minus electrode 123b. The tilt mirror 120 has electrostatic capacity Ca between the mirror 121 and the plus driving electrode 123a, and electrostatic capacity Cb between the mirror 121 and the minus driving electrode 123b. When a driving voltage on a plus driving electrode or a minus driving electrode are supplied to the tilt mirror 120, the angle of the mirror 121 can be changed continuously depending on values of the driving voltages, around the indrive position where the driving voltages are 0 V. When the angle of the mirror 121 is changed continuously, values of the elctrostatic capacities Ca and Cb change continuously correspondingly to the angle change. The angle of the mirror 121 is controlled by a driving unit.
FIG. 25 is a block diagram showing a conventional driving unit for an optical switch. An input signal corresponding to an angle for changing the angle of the tilt mirror 120 through the digital filter 130 is D/A converted by the D/A converter, and then is amplified to high voltage by the high-voltage amplifier 132 and is supplied to the tilt mirror 120 to drive it. The digital filter 130 is a band elimination filter (BEF) for limiting gains near the resonance frequency element of the input signal in order to reduce the self-resonance phenomenon of the mirror 121. This BEF is constituted by an FPGA, a DSP, etc. The driving unit shown in FIG. 25 is of a constitution of a feedforward control system outputting a driving voltage on the basis of an input signal, and is able to continuously change the angle of the mirror 121 corresponding to values of driving voltages output by supplying input signals corresponding to variations of the angle.
Next, a large-scale optical switch composed of a tilt mirror to which a MEMS technology is applied will be described.
As described above, in recent years, data traffic in various networks has been increasing explosively, and it has been developed to construct a photonic network capable of processing a large amount of data traffic. Furthermore, it is expected that various networks and photonic networks would develop in a mesh configuration in the near future. In order to operate a mesh-like photonic network with flexibility, nodes of the network need a function of exchanging (cross-connecting) desired paths (routes). Realization of a large-scale optical cross-connect requires exchange of paths whose number is decided by connection nodes (e.g., 10 nodes)×wavelength (e.g., 40 waves), and requires, for this purpose, a large-scale optical switch up to hundreds to thousands. For this large scale cross-connecting function, an optical switch to which a 3D-MEMS technology is applied is most suitable.
FIG. 21A is a block diagram of a conventional optical switch controller (3D-MEM switch controller). The controller 47a shown in FIG. 21A sets voltage data to the angle of the mirror on the basis of a tilt-angle setting table which defines correspondences between angles (θ°) of the mirror (MEMS mirror 33) and voltage data (V) representing a driving voltage. The voltage data is given to the D/A converter 12 and the MEMS driver 15a to drive the mirror. Concurrently with driving the mirror angle feedback control using an angle sensor is performed, and the controller 47a performs PID control on the basis of the angle data fed back to perform correction of deviation of the driving voltage, suppression of the mirror resonance phenomena, etc. The mirror, which has been set at a predetermined angle, deflects and switches signal light input from an optical fiber.
A technique relative to this angle feedback control has also been proposed (see non-patent document 1).
For angle detection using an angle sensor, an electrostatic capacity sensor (amplifier-type capacity sensor 47b), for example, is used. In this electrostatic capacity sensor, the amplification factor of a detection signal changes depending on the tilt of the mirror on the basis of variations of electrostatic capacities caused by the tilt of the mirror and the operation of driving electrodes depending on the tilt of the mirror. That is, the amplitude of the detection signal increases or decreases with the tilt of the mirror. Since the detection signal output from this electrostatic capacity sensor is an analog signal, the detection signal is converted to a digital signal through a sample-and-hold circuit 47c and an A/D converter 12a to perform digital control.
As described above, when a large-scale optical cross-connecting device is realized using a 3D-MEMS optical switch, necessary number of optical switch controllers is hundreds to thousands as an example.
In response to this large-scale need, techniques of integrating angle sensors (electrostatic capacity sensors) on a MEMS chip and the like have been developing.
Non-patent document 1: “High-speed Switching Three-dimensional MEMS Optical switch” Communication Society Conf. of Electronic Information Communication Institute, pp. 447, 2002
Non-patent document 2: Brener et al. “Nonlinear Servo Control of MEMS Mirrors and Their Performance in a Large Port-Court Optical Switch”, Optical Fiber Communication Conf. 2003, Atlanta, Ga., 2003.
However, residual vibration (amplitude) occurs on the tilt mirror 120 due to the self-resonance phenomenon of the mirror 121 when the mirror 121 is tilted to a desired angle θ. The tilt mirror 120 has electrostatic capacities as described above. The equation of motion of the tilt mirror 120 is shown as equation (1) in which I is the moment of inertia, c is the attenuation coefficient, k is the spring constant, C is the electrostatic capacity of the tilt mirror, and V is the driving voltage.
                                          I            ⁢                                                  ⁢                          θ              ″                                +                      c            ⁢                                                  ⁢                          θ              ′                                +                      k            ⁢                                                  ⁢            θ                          =                              1            2                    ⁢                                    ∂              C                                      ∂              θ                                ⁢                      V            2                                              (        1        )            
Like this, the ultimate angle of the tilt mirror changes corresponding to variations of the electrostatic capacity of the tilt mirror. FIG. 26 is a graph showing the characteristic between the electrostatic capacity of the mirror and the rotational angle. The horizontal axis indicates the rotational angle θ, and the vertical axis indicates the electrostatic capacity C. As shown in the figure, the mirror 121 has a characteristic that the electrostatic capacity is proportional to the rotational angle (linearity) in the plus angle region (tilts shown with a solid line in FIG. 24), and is not proportional to the rotational angle (nonlinearity) in the minus angle region (tilts shown with a dotted line in FIG. 24), with the boundary where the mirror is in a horizontal position (0°). Furthermore, since the mirror 121's own Q value is large, residual vibration occurs due to the self-resonance phenomenon. Because of this, a continuous feedforward control system using the driving unit described above has a problem that the residual vibration can not be restricted when the angle of the mirror is changed particularly to a minus angle. An optical switch composed of such a tilt mirror reduces the accuracy and response speed of angle control in changeover of optical paths of optical transmission lines.
In this configuration, in the feedforward control system using the driving unit described above, the Q value needs to be reduced in order to restrict the self-resonance phenomenon of the mirror 121. When the Q value is reduced, the residual vibration caused by the self-resonance phenomenon can be reduced, but the response speed of angle control of the mirror 121 is reduced. By increasing the Q value, the response speed of the angle control can be increased, but the residual vibration caused by the self-resonance phenomenon of the mirror 121 deteriorates (increases).
Like this, in order to restrict the self-resonance phenomenon of the mirror 121, it is necessary to optimize the parameters of the digital filter 130 and improve the slew rate, etc. of the high-voltage amplifier which affect the angle response control of the mirror 121, but these require much time and labor in adjustment, and can not eliminate the self-resonance phenomenon arising at angle change, and increase the response speed. A tilt mirror is taken as an example in the above description, but the same problem arises also when controlling, in the same way, the movement of a micro movable body using a MEMS technology, etc.
Furthermore, the A/D converter 12a provided in the optical switch controller shown in FIG. 21A is large in circuit scale and power consumption in general, and has a characteristic of being weak against crosstalk between channels, and the like because it is an analog circuit, thereby being a bottleneck in realization of a large-scale optical cross-connecting device. The reason is that since the angle sensor is an analog circuit, complex analog processing such as A/D conversion is required, and thereby a large-scale optical switch can not be realized.
Conventionally, it results a large-scale circuit that an IC (Integrated Circuit) containing hundreds of A/D converters is mounted on a board or substrate for the purpose of realization of a large-scale switch, and it requires a large chip area that the A/D converters are integrated in a controller IC or a controller LSI (Large Scale Integration) for the same purpose, and therefore there is a problem that it is impossible to realize a large-scale switch.